Alloys resistant to high temperatures



United States Patent C 3,220,828 ALLGYS RESISTANT TO HIGH TEMPERATURESWilliam T. Kaarlela, Fort Worth, Tex., assignor to General DynamicsCorporation, San Diego, Calif, 21 corporation of Delaware No Drawing.Filed Apr. 9, 1963, Ser. No. 271,577 3 Claims. (Cl. 75134) Thisinvention relates in general to refractory metal alloys capable ofresisting high temperatures and more particularly, to alloys of suchcharacter as may be well suited for the brazing of refractory metalssuch as columbium, tantalum, molybdenum, and their alloys, and havingadditional utility as high temperature protective coatings and asstructural alloys.

It has become increasingly important, particularly in missile,spacecraft and aircraft applications, to use materials which are capableof withstanding extremely high temperatures. In addition to molybdenumand tantalum, one such material is columbium, which is a refractorymetal possessing a melting point in the vicinity of 4400 F. From adesign standpoint, it is an excellent material because of its highstrength to weight ratio in the 2000 to 2500 F. range of servicetemperatures. It does not strain-harden rapidly; allowing cold workingup to 99% Without annealing, and it is therefore particularly suitablefor the forming of parts of complex shape. Columbium is alsocharacterized by moderate density (comparable to iron and nickel), and ahigh melting point, with good strength retention above the useful rangeof currently available alloys.

Useful though it is in high temperature areas, joinder of the metal andits alloys present problems. Thus, although it may be welded, nitrogencontamination is a deleterious obstacle since it causes an increase inthe tendency for crater cracking, serious loss of ductility and anincrease in the transition temperature. Welding also presents theproblem of loss of strength due to recrystallization. In the handling ofhigh strength columbium alloys, recrystallization occurs between 2200and 2800" F., depending upon the alloy makeup. Welding involves thesehigh temperatures and where recrystallization as a result occurs, a lossof approximately 50% in tensile strength may be anticipated.

However, it has been found that by using the alloys of this invention,excellent joinder of the metals is effected and a high temperatureresistant joint is produced which is compatible with the strengthcharacteristics of the joined materials. Vacuum environment or inertatmospheres such as argon gas are utilized to prevent oxidation of thecolumbium and its alloys during the brazing; only moderate efforts beingnecessary for purification of the argon gas preparatory to brazing.

The alloys of this invention are additionally useful as coatings forcolumbium and for other materials characterized by low oxidationresistance at elevated temperatures, i.e., of approximately 2000 F.

Further utility for these alloys is found in structural applications,such as castings, and in other areas of service where adequate oxidationresistance at elevated temperatures is difficult to achieve and maintainand where both structural and non-structural provisions are itrequirement.

Accordingly, it is an object of this invention to provide alloys wellsuited for the brazing of refractory metals such as columbium,molybdenum, tantalum, and their alloys; which alloys are capable ofproviding excellent joint strength at elevated temperatures.

It is another object of this invention to provide alloys of thecharacter described which do not require excessively high temperaturesfor brazing and which possess adequate ductility.

A further object is to provide alloys, as described, which do not causeexcessive erosion of the base alloys when applied thereto in brazingapplications.

Yet a further object is to provide alloys suitable as protectivecoatings for preventing oxidation of materials having a highsusceptibility thereto at elevated temperatures.

Another object is the provision of alloys adapted to castingapplications which call for materials possessing adequate oxidationresistance at elevated temperatures.

These and other objects and advantages of this invention will becomeapparent from the following description of the alloys and theircharacteristics and the claims directed thereto.

In general the alloys of this invention include titanium as a matrixelement in the percentage-by-weight ranges indicated. This matrixelement is characterized by its compatibility with columbium,molybdenum, tantalum, and the alloys thereof. At least one alloyingelement selected from the group consisting of chromium, palladium, andat least one selected from the group consisting of silicon, andgermanium and proportioned as set forth, is added to the matrix element.The resulting alloys have produced brazed joints possessing jointstrength at high temperatures. This brazing has been effected withoutthe use of extremely high temperatures. Ability of the alloys towithstand high temperatures with a minimum of deterioration hascontributed to their further utilization as coatings and as barriers forprotection against oxidation of various materials inherently susceptiblethereto. Usable tensile strength of these alloys at temperatures around2000 F. further dictate their use in the area of structural material,particularly in casting applications.

It is to be understood that percentages used herein in bothspecification and claims, to describe ingredient proportions, arepercentages by weight unless otherwise stated.

Titanium as an elemental constituent to each of the alloys has beenfound to be quite compatible with the refractory metals forming a tough,narrow, diffusion layer at the interface with the brazing alloy. Asemployed herein, the general range of titanium is from about 20% toabout 90% by weight of the alloy. A preferred range has been found to befrom 30% to about Superior alloy compositions have been establishedincorporating the specific proportions of titanium tabulated below.

Chromium, like titanium, is quite compatible with the refractory metalsforming a diffusion layer at the interface with the brazing alloy. Asemployed herein the general range for chromium is from about 20% toabout 65% by weight of the alloy. A preferred range has been found to be30% to about 48%. Superior alloy compositions include chromium in thequantities set forth hereinafter.

Palladium, like chromium and titanium, serves as a matrix element incertain of the present alloys. Like chromium, it has been found to bequite compatible with columbium and forms an interface alloy layer. Itenters into the alloys of this invention in the general range of fromabout 5% to about 35%. Its preferred range is from about 10% to about30%. Superior alloy compositions include palladium in the quantities setforth hereinafter.

Germanium serves to improve oxidation resistance and depresses themelting point, while silicon increases the high temperature shearstrength and also serves as a melting point depressant. The generalrange for silicon is from about 5% to about 25%; the preferred rangebeing about 6% to about 14%. For germanium, the general range is fromabout 5% to about 25%, and the preferred range is from about 7% to about16%.

Although the process for alloy formulation is subject to considerablevariation, the alloys of the present invention have, for test purposes,been formulated by mixing the elemental ingredients together in thedesired proportions Further, each alloy was tested for structuralintegrity, brazing characteristics and environmental resistance. Suchvalues as brazing temperature, re-melt temperature, shear strength,toughness factor and brazeability on in powder form. The mixture issubsequently briquetted 5 columbium, molybdenum and tantalum wereestablished into a compact; then melted in a cold-hearth, waterand areset forth in tabular form below:

Joint Remolt Shear Strength in Brazeability 0n- Brazo Temp. F. on Chjoint in p.s.i. Tough- No. Temp. ncss in F. Index" Cb Ta 72 2,000 Cl) M0Ta 2,500 3,300 +2 800 45, 000 7, (300 2,050 C-cod Excellent. Good. 2,700+3,500 25,000 23,800 1, 920 Excellent Good"...

700 +3, 500 24,000 23,100 Excollent. Good"..-

*Value of 2,000 or higher indicates good toughness.

cooled, copper crucible. If ductile, the alloy is then rolled to form afoil, or in the alternative, broken and crushed into a powder to be usedin this form.

As formulated herein, the alloys of this invention have taken the powderform. Application is effected by mixing the powder alloy with polyvinylalcohol in a slurry, which is then painted on the joint to be brazed.The alloy is then heated to a temperature above its melting point in aprotective environment, such as in a vacuum or using an argon atmosphereor other suitable inert gases for protection against the detrimentaleffect of oxidation. As hereinabove stated, moderate efforts should bemade to purify the argon gas, it best results are to be obtained. Thishas been accomplished satisfactorily, in the present instance, bypassing the argon through a 100 F, Dry Ice-acetone cold trap, and aclosed zirconia tube filled with titanium strips and operating at 1750F. The protection offered by the inert argon gas is important. Shouldatmospheric contamination occur it will be directly reflected in reducedflow and wetting of the brazing alloy, resulting in an inferior joint.

For purposes of the tests, the results of which are reflected in thetabulations below, brazed lap shear test specimens were made up using acolumbium alloy incorporating by weight titanium and 10% molybdenum.These were employed as the members to be joined and lap shear tested.Time at temperature prior to testing was 1-5 minutes. Employing anA-frame type lap shear tension test apparatus, failure was made to occurwithin one minute by steadily increasing the mechanical stress upon thespecimen by means of a floating screw. Test temperatures were effectedby the induction heating of a graphite susceptor surrounding thespecimen. Stress was measured by means of a calibrated load link inconjunction with a strain recorder. Specimens were confined underprotective, substantially inert, argon atmosphere during heating,testing and cooling.

Each of the alloys of this invention is set forth in the table belowwith an indication both as to the general range of its ingredients andas to the specific composition of the particular alloy or alloys tested:

General Range or Specific Composition Ti Or Ge {Specific During thesetests, a remelt temperature rise phenomenon was observed in connectionwith alloys designated Numbers 1, 2, 3, in the above tabulation of testresults. That is to say the brazing temperature of these particularalloys ranges between 2500 F. and 2700 F. However, in tests afterbrazing of the columbium or tantalum T specimens, it was found thatthese alloys did not remelt between 2800 and more than 3500 F., thehighest temperature checked. Although similar tests were not conductedwith respect to the other of the enumerated specimens, it is believedthat a similar phenomenon would be found to exist for each of them.

I claim:

1. An alloy characterized by its ability to withstand high temperaturesand consisting of from about 20% to about 60% titanium, from about 20%to about 60% chromium, from about 5% to about 35% palladium, and fromabout 5% to about 25% silicon.

2. An alloy characterized by its ability to withstand high temperaturesand consisting of from about 30% to about 48% titanium, from about 30%to about 48% chromium, from about 10% to about 30% palladium, and fromabout 7% to about 12% silicon.

3. An alloy characterized by its ability to withstand high temperaturesand consisting of about 35% titanium, about 35% chromium, about 20%palladium, and about 10% silicon.

References Cited by the Examiner UNITED STATES PATENTS 2,169,193 8/1939Comstock -134.3 3,111,406 11/1963 Kaarlela 75-175.5 3,131,059 4/1964Kaarlela 75-176 OTHER REFERENCES Hansen, M., Constitution of BinaryAlloys, 2nd Ed.,

McGraw-Hill, New York, 1958, pages 565568.

DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner.

1. AN ALLOY CHARACTERIZED BY ITS ABILITY TO WITHSTAND HIGH TEMPERATURESAND CONSISTING OF FROM ABOUT 20% TO ABOUT 60% TITANIUM, FROM ABOUT 20%TO ABOUT 60% CHROMIUM, FROM ABOUT 5% TO ABOUT 35% PALLADIUM, AND FROMABOUT 5% TO ABOUT 25% SILICON.